Guppies sexually harass threatened species

Male guppies may sexually harass females of another fish species to prevent them from reproducing, researchers suggest. They believe the guppies - which have invaded Mexican rivers and lakes - are using sex as a way of suppressing one native fish population.

The researchers also think the guppies may physically harm the native females so they are unable to reproduce with males of their own species, or shy away from further interactions with males.

Guppies, originally from Trinidad, invaded Mexican waters in the 1950s. The pet trade is generally blamed for introducing them around the world as aquarium owners wanting to get rid of fish, sometimes dump them into rivers.

In Mexico and elsewhere, the released guppies rapidly reproduced. "Male guppies are very sexually active," explains Alejandra Valero at the Universidad Nacional Autónoma de Mexico, who led the new study. She says guppies are "like sex machines".

Declining species

One species that suffered from the invasion of the guppies was Skiffia bilineata, a fish native to Mexican waters which is threatened with extinction. Female skiffia look like female guppies, so Valero and her colleagues wanted to find out if this was contributing to the species' decline.

They put male and female guppies in aquariums with female skiffia and found that no matter how many female guppies were around, male guppies would try to copulate with females of both species.

Their advances were unsuccessful, but Valero and her colleagues believe that the attempts at sex might be harming the female skiffia. If this is true, the skiffia may not be able to go on to reproduce with males of their own species.

The two species have very different ways of reproducing. Skiffia sex is consensual - males have no extending reproductive organ so the only way for their sperm to reach the females' eggs is for the two animals to line up their genital openings.

Sexual harassment

Guppy sex is more violent. Males have a hooked genital organ known as a gonopodium, which they insert into the females. Past research has shown that the gonopodium maims guppy females. It is thought that the resulting inflammation locks the sperm inside the female.

Valero and her colleagues believe that the guppies may attempt to insert their hooked organ into female skiffia and harm them in the process.

"We see the males draw their gonopodium forward and try to insert it into the genital pore of the Mexican females," says Valero. "In some cases, we think they did insert it, because we saw the skiffia females jerking away."

Sexual harassment of native species by an invasive species has been suggested before in a very different animal - the mink. "The story is intriguing but slightly speculative," cautions David Macdonald, director of the Wildlife Conservation Research Unit at Oxford University in the UK.

American mink, which escaped from fur farms and invaded the European and Russian countryside, are the main threat to the endangered European mink.

'Pointless' reproduction

Although evidence overwhelmingly suggests that the American mink brutalise the native species and have driven them to near-extinction through violent interactions, it is possible that male American minks may also attempt to reproduce with female European minks.

As with the guppies and skiffia, the females of the two species look similar. "There is tentative evidence that this mating has led to non-viable hybrid offspring," says Macdonald. If this is indeed happening, such "pointless" reproduction would reduce the number of European mink.

Valero and her colleagues now want to determine whether such interactions do leave female skiffia wounded and if they affect their future reproductive success.

"For me, the most interesting question is whether Mexican females respond differently to their own males after being harassed by guppies?" Valero says. "Do they then 'fear' males?"

"We now know harassment between very different species exists," she says. "It is happening in lab and, because the species live in the same spots, it is definitely going on in nature too."

Endangered species - Learn more about the conservation battle in our comprehensive special report.

If you would like to reuse any content from New Scientist, either in print or online, please contact the syndication department first for permission. New Scientist does not own rights to photos, but there are a variety of licensing options available for use of articles and graphics we own the copyright to.